1. Field of the Invention
The present invention relates to the art of earth boring with rolling cutter drill bits. In particular, the present invention relates to a design of, and a method for making, a rolling cutter drill bit to provide for better retention of cutting inserts disposed in the rolling cutter of the drill bit.
2. Description of Related Art
Rolling cutter drill bits have cutting teeth on the rolling cutter to crush and penetrate the rock being drilled. There are two basic types of rolling cutter drill bits. The first type has steel teeth formed on the rolling cutter. These teeth usually have a layer of hard wear, resistant material to resist the abrasive action of the rock.
The second type of rolling cutter drill bit has steel rolling cutters with a number of protruding tungsten carbide cutting inserts. Machined upon each cutter are one or more conic segments with different conic angles. These segments form bands, or conic bands, which are concentric with the longitudinal axis of the cutter. The cutting inserts typically have a generally cylindrical shape and a generally circular cross-sectional configuration. The cutting inserts are received by sockets, or insert holes, drilled into the rolling cutter, perpendicular to the surface of the conic band, or conic segments. A plurality of insert holes drilled along the same radius of a band to form a row of insert holes on that band. The cutting inserts are held in place in the sockets by an interference fit. Although the bands are curved, when viewed in a cross-section parallel to the axis of the cutter, the upper surfaces of the bands are flat. For clarity in understanding the present invention, the flat cross-section of the band upper surface is defined as the axial surface of the band.
To provide an interference fit between the cutting insert and the socket, the socket is formed with a diameter slightly smaller than that of the cylindrical body of the cutting insert. The cutting insert is then pressed into the socket and retained by the contact force between the socket wall and the outer wall surface of the cylindrical body of the cutting insert. Because the insert diameter exceeds that of the socket and because of the hardness of the cutting insert material, the installation procedure can be difficult and can damage the socket. A damaged socket can reduce the contact force between the cutting insert and the socket wall. If the socket becomes sufficiently damaged during installation, the cutting insert can dislodge from the socket during drilling operations. Additionally, a damaged socket can also allow the cutting insert to rotate in the socket during drilling, which can decrease the cutting effectiveness of the cutting insert. Traditional approaches to solve the foregoing problems have generally included modifying the shape of the cutting insert and/or modifying the shape and taper of the socket.
Since the conic band on the cutter is curved, the insert can move out of axial alignment with the insert hole as it is being pressed into the insert hole. One possible technique to prevent this mis-alignment is to spotface a flat area on the surface of the cutter around each insert hole, or socket, with a milling machine. The juncture of the outer surface of the cutter and the insert hole will then form a plane, which will fully contact the periphery of the bottom of the insert. This full contact could help to minimize mis-alignment of the insert with respect to the insert hole during pressing.
The process of spotfacing the curved surface around each insert hole is not only time consuming and expensive, but this process may also leave sharp edge surfaces that can become sites for undesired crack formation. Consequently, the integrity of the insert press fit may often be less than ideal. A poor press fit can cause an insert to loosen in operation and fall out of the insert socket, or insert hole, of the cutter.
Accordingly, prior to the development of the present invention, there has been no rolling cutter drill bit with stabilized insert holes or method for making a rolling cutter drill bit with stabilized insert holes, which: routinely and consistently permit cutting inserts to be pressed into the rolling cutter without misalignment problems; prevent the cutting inserts from prematurely being loosened and removed from their sockets; prevent the cutting inserts from rotating within their sockets; and are efficiently and inexpensively utilized. Therefore, the art has sought a rolling cutter drill bit with stabilized insert holes and a method for making a rolling cutter drill bit with stabilized insert holes which: routinely and consistently permits cutting inserts to be pressed into their sockets without misalignment problems; prevent the cutting inserts from prematurely loosening within their sockets and falling out of the sockets; prevent the cutting inserts from rotating within their sockets; and are efficient and economical to manufacture and use.